Method for increasing the heating rate in continuous annealing processes

ABSTRACT

IN THE CONTINUOUS ANNEALING OF STEEL STRANDS, THE HOT ROLLED COIL IS PASSED TO A HIGH SPEED TANDEM MILL WHERE IT IS COLD REDUCED. A WATER SOLUBLE ROLLING LUBRICANT IS EMPLOYED TO BOTH REDUCE THE ROLLING FORCES AND TO SUBSEQUENTLY ACT AS AN EMISSITY INCREASING FILM. THE STEEL STRAND, WITH THE FILM OF LUBRICANT (0.5 TO 10.0 MICROINCHES THICK) ADHERING TO ITS SURFACES, IS PASSED TO A RADIANT HEATING, ANNEALING FURNACE. SUBSEQUENT TO ANNEALING, THE STRAND IS COOLED AND THE LUBRICANT REMOVED BY WELLKNOWN MEANS.

United States Patent 3,725,140 METHOD FOR INCREASING THE HEATING RATE IN CONTINUOUS ANNEALING PROCESSES Edward J. Patula, Monroeville Borough, William L. Roberts, Franklin Township, Westmoreland County, and Robert R. Somers, Penn Hills Borough, Pa., assignors to United States Steel Corporation No Drawing. Filed Mar. 29, 1971, Ser. No. 129,183 Int. Cl. C2111 7/00 US. Cl. 148--12.1 1 Claim ABSTRACT OF THE DISCLOSURE In the continuous annealing of steel strands, the hot rolled coil is passed to a high speed tandem mill where it is cold reduced. A water soluble rolling lubricant is employed to both reduce the rolling forces and to subsequently act as an emissivity increasing film. The steel strand, with the film of lubricant (0.5 to 10.0 microinches thick) adhering to its surfaces, is passed to a radiant heating, annealing furnace. Subsequent to annealing, the strand is cooled and the lubricant removed by wellknown means.

This invention relates to a method for the continuous annealing of steel strand material, whereby both the heatup rate and cooling rate of the material undergoing recrystallization is markedly increased.

Continuous annealing is employed extensively in the production of recrystallized sheet and strip material. In this process, a hot rolled coil is fed through a high-speed tandem mill, to cold reduce the coil to the desired gage. These high-speed mills necessitate the use of rolling lubricants to decrease the required rolling force and to serve as a coolant for both the rolls and the strip. After cold reduction, the strip is cleaned so that any dirt and lubricant adhering to its surface is thoroughly removed. The cleaned strip then travels at high speed through a radiant heating zone, under a non-oxidizing atmosphere, where it is brought to desired temperature in a short time, held at temperature for a period of a few seconds, passed through a cooling zone, and then emerges into the air at temperatures below that at which it will rapidly oxidize.

Since both the heating and cooling of the strip are effected primarily by radiation, the heating and cooling rates are dependent on the emissivity of the surface of the strip. An ideal black body has an emissivity of 1.0, whereas the ordinary, cleaned steel strip employed in such continuous annealing processes has an emissivity below 0.2, thereby resulting in much slower heat transfer rates than could be obtained with surfaces having higher emissivities. Consequently, a considerable length of heating and cooling section is required so that the strip may achieve the desired temperature; and for commercial lines operating at speeds of about 1500 ft./min., these sections are necessarily large and expensive. If the emissivity of the strip could be effectively increased prior to its entry into the heating zone of the furnace, continuous annealing lines operating at a fixed speed could be reduced in size and cost. Conversely, lines could be made to operate at even higher speeds and on thicker strip.

In the process of this invention, this desirable increase in surface emissivity is achieved by passing the strand, with a uniform, thin film of rolling lubricant still adhering to its surface, directly into the annealing furnace. Following recrystallization, the strand is cooled with the lubricant film intact, after which it is cleaned by well-known methods such as scrubbing and electrolytic cleaning. For purposes of this invention, the term strand as used herein and in the appended claims is meant to include all flat-rolled metal articles such as strip, sheet, wire, rod

3,725,140 Patented Apr. 3, 1973 or other such elongated articles amenable to continuous radiative heat treatment.

In continuous annealing processes, a typical arrangement for removal of rolling lubricants from strip issuing from the high-speed cold reduction mill would involve the following stages:

(a) a pre-dunk tank, containing an alkaline cleaning solution for removal of dirt and excess rolling oils;

(b) a #1 brush scrubber;

(c) an electrolytic cleaning operation utilizing a grid cleaning method;

(d) a #2 brush scrubber, essentially a duplicate of the #1 scrubber, which performs the final scrubbing action;

(e) a rinse tank, utilizing either spray or dip rinsing to removed cleaning salts;

(f) wringer rolls to prevent excess water from being carried along, and thereby not effectively removed by (g) hot-air dryers.

In the instant process, these steps may all be eliminated and the strand passed, directly from the tandem mill, to the annealing furnace. To provide the desired increase in heat-up rate, the lubricant film should be spread uniformly over the entire surface of the strand, and should preferably have a thickness of between about 0.5 to about 10.0 microinches. In commercial practice, the residual oil film exiting from the tandem mill (prior to the above cleaning stages) will generally have a uniform thickness of from about 0.7 to 3.0 microinches. Therefore, since the cold reduction rolls will generally provide residual oil-films of a thickness within the prescribed limits (0.5 to 10.0 microinches), it will generally not be necessary to pass the strand to any additional processing, prior to its entrance into the annealing furnace. However, if, for some reason, the residual film thickness falls outside of the desired range, it may easily be regulated by small variations in the cold-rolling procedure. Thus, the residualfilm thickness may be decreased by one or more of the following: (1) an increase in the specific rolling force; (2) an increase in mill speed; (3) an increase in the percent reduction; (4) an increase in the thickness of the initially applied oil-film; and (5) a change in the type of emulsion employed. In an alternate, less preferable procedure, the strip may be passed to one of the above cleaning stages to remove excess oil and dirt. However, if such a cleaning stage is employed, it would of course be necessary to further rinse and dry the strip.

Although essentially any rolling lubricant will provide some increase in the emissivity of the steel strand, it is desirable that in addition to (a) effectively increasing the heat-up rate, the lubricant possesses the following additional characteristics; (b) it should be water-dispersible, both before and after annealing, so that it may be easily and economically removed; (c) it should not react with the steel surface, so as to discolor or oxidize the strand; (d) it should have a high boiling point and no tendency to decompose at annealing temperatures, e.g., up to about 1300 F.; (e) it should not otherwise react with the steel surface, so as to impair a subsequent plating operation.

Since lubricant vendors carefully guard their formulations, it is diiiicult to clearly specify (other than by their proprietary names) those lubricants which possess an optimum combination of the above stated properties. However, by employing a relatively simple test procedure, such as outlined below, a particular rolling oil may be easily evaluated to determine its utility and practibility for the purposes of this invention.

Potential rolling oil coatings were evaluated using a muffle furnace with a specially designed protective atmosphere chamber. Oils were evaluated for their case of removal and their effect on strip surface, as well as for their effectiveness in increasing emissivity. This latter parameter was indirectly measured, by determining the average heating rate of the steel, from 100 F. to a temperature of 1100 F., in a furnace maintained at about 1400 F. In the heating section of the furnace, the samples -(4 in. x 8 in. x 0.0085 inches thick) were inserted between, and parallel to, two heated graphite plates. These plates were heated by radiation from the furnace walls and in turn, imparted heat to the samples, substantially by radiation. Thermocouples were attached to separate samples of both coated and uncoated strips, so that the temperatures of both could be simultaneously and continuously recorded. By comparing the heating rates of the coated to the uncoated strip, the relative effect of a coating material on strip emissivity could be determined. After the samples were annealed, they were cooled in a separate sealed section of the chamber which was also under a protective nitrogen atmosphere. The samples were then removed and examined visually for contamination by oxidation or other staining. An attempt was then made to clean the strip by scrubbing with detergent and water.

Of the lubricants tested, increases in heat-up rates of greater than 100% were achieved. A few representative samples (of different proprietary compositions) are given NOTE-These samples represent the products of four different manufacturers, with Nos. 11, 18, and 19 being from the same manufacturer.

Of the above examples, numbers 11, 13, and 23 were easily removed by simple scrubbing and showed no surface contamination. Thus, while some rolling lubricants will provide significant increases (e.g., sample No. 19, greater than 100%) in heat-up rate, they may be less desirable in that they have a deleterious effect on surface characteristics and/or are more difiicult to remove. In some instances, slight discoloration may be tolerable and then those oils which provide the higher emissivity increases could be employed. The determination of which lubricant to employ will ultimately depend on economic considerations, balancing the relative efficiency of the oil as a rolling lubricant and as an emissivity coating, against the difierences in cost of removal subsequent to annealing.

We claim:

1. In a process for continuously annealing cold reduced steel strand, a method for increasing the heat-up rate of said strand which comprises:

(a) in a cold reduction stage, employing a rolling oil which does not substantially vaporize or decompose at temperatures up to about 1300 F., so as to both decrease the requisite roll separating forces and to coat the surface of said strand with an oily film;

(b) feeding the cold reduced strand, with said film of oil having a thickness of from 0.5 to 10.0 microinches uniformly covering the surface thereof, to a continuous annealing stage, wherein the temperature of said strand is elevated, substantially by radiative heating, to a temperature above the lower critical;

(c) subsequent to said annealing, decreasing the temperature of said strand in a cooling stage, in which at least the first portion of said cooling is achieved substantially by radiation; and

(d) cleaning the resultant annealed strand to remove said oily film.

References Cited Bastian, E. L. H.; Metalworking Lubricants-Their Selection, Application, and Maintenance; McGraw-Hill,

WAYLAND W. STALLARD, Primary Examiner U.S. Cl. XR. 

